Principal Investigator/Program Director: Evans, Christopher J., Ph.D., Component IV Component IV: Defining Distinct Roles of the Striatonigral and Striatopallidal Neurons in Opiate Reward ABSTRACT: Converging molecular, genetic and pharmacological evidence suggest that an important part of the brain reward circuit that mediates all forms of drug rewards are the Striatonigral and Striatopallidal projection neurons in the nucleus accumbens and the dorsal striatum. Recent advance in Bacterial Artificial Chromosome (BAC)-mediated transgenesis has for the first time allowed us to genetically- label the mosaically distributed Striatonigral and Striatopallidal neurons for molecular analyses. In our preliminary study, we have developed a novel method called FACS-array, which allows purification of genetically-labeled Striatopallidal or Striatonigral neurons from the adult mouse brains for cell-type-specific gene expression profiling. In this component, we hypothesize that the Striatonigral and Striatopallidal neurons play critical but distinct roles in opiate reward. To test this hypothesis, we propose the following experiments. First, since the Striatonigral neurons in the patch compartment express very high levels of nOR,and because accumulating evidence suggest that opiates may directly act within the nucleus accumbens to establish reward, we hypothesize that the direct actions of opiates on the Striatonigral neurons may be sufficient to establish reward. To test this idea, we will use a BAG transgenic approach to selectively express i*OR in the Striatonigral neurons in the patch of the pOR knockout mice in order to test for restoration of opiate reward in vivo. Secondly, since both the Striatonigral and Striatopallidal neurons are shown to be involved in all forms of drug rewards, to identify the critical cell-type-specific mediators of opiate reward, we will apply the FACS-array technology to identify the Striatonigral and Striatopallidal circuit specific gene expression changes elicited by chronic morphine administration, and then use genetic mutants that are lacking opiate reward (i.e. /nOR and Drd2 knockout mice) to uncover the gene expression changes that are associated with opiate reward. Our genetic analyses may help to define a minimal neural circuit that is directly targeted by opiates to elicit reward, and to identify a critical set of circuit-specific molecular changes underlying opiate reward and addiction. Detailed mechanistic understanding on how drugs of addiction interact with genes within the brain's reward circuits may accelerate the development of novel therapeutics to prevent and/or to treat addiction. Primary